The murine major histocompatibility complex, H-2, serves as an experimental model in which to investigate immunoregulatory mechanisms governed by this tightly-linked gene cluster of higher vertebrates. The critically important I-region of H-2, defined by immune response genes, also controls T-B cell and T-macrophage interaction, graft-versus-host reactivity, virus susceptibility, immunosuppression, and other traits. I-region-associated antigens (Ia) are considered seeks as a long-term objective to explore a possible structure-function relationship between immune response region-associated (Ia) antigens on T lymphocytes, and immunoregulatory gene expression. The specific aims of experiments proposed herein include: 1) To produce anti-Ia serum directed against T lymphocyte antigens encoded by I subregions A. B. J. E. and C by immunization of I-region congenic strain pairs with congenic strain pairs with concanavalin A-stimulated thymocytes. 2. To analyze T cell-specific Ia antisera by standard serological methods (dye exclusion microcytotoyicity, radioimmunoprecipitation, absorption) with regard to cellular expression (thymocytes, peripheral T cells and others) and haplotype crossreactivity of antigenic determinants. 3) To determine which immune functions (i.e., cell-mediated cytolysis, are carried out by Ia antigen-bearing T lymphocytes. 4) To attempt to isolate (by detergent extraction, affinity and conventional chromatographic separation) and biochemically characterize (chemical composition, biophysical characteristics and immunochemical properties) a T cell-specific Ia antigen-bearing molecule.